The invention relates to a composite body composed of polyacetal and of at least one thermoplastic vulcanizate (TPV) elastomer (=TPV), and also to a process for its production, where modification of the TPV elastomer with non-olefinic thermoplastic materials enabled an adhesive or cohesive bonding to be obtained between the polyacetal and the TPV. Use of specific crosslinking agent systems can avoid degradation of the polyacetal.
Polyacetal, i.e. polyoxymethylene (POM), is an engineering material with excellent mechanical properties and is also moreover generally resistant to all of the usual solvents and engine fuels. Good strength and hardness associated with excellent rebound resilience means moldings composed of polyacetal are very frequently used for snap connections, in particular clips, in every sector of everyday life. Excellent sliding friction properties are the reason for use of POM for many movable parts, such as transmission parts, deflector rolls, gearwheels, or shift levers. Moldings composed of POM are also very frequently used in automobile construction. Very good mechanical strength and chemicals resistance mean that POM is also used to produce a wide variety of housings and keyboards.
However, POM has a low mechanical damping factor at room temperature, and in some applications this necessitates use of soft damping elements. When moldings composed of POM are installed, connection sites also often require a seal. The high surface hardness of moldings composed of POM and the low coefficient of sliding friction of POM can lead to slip of superposed articles and can create risk in the operation of, for example, switching elements and control elements composed of POM. On the other hand, it is true that combinations composed of hard and soft materials are used with increasing frequency in order to achieve a mutual combination of the particular properties of these materials. The hard material here is intended to give the components their strength, and because the soft material has elastic properties it assumes functions related to sealing or vibration-damping and sound-deadening, or brings about a change in surface feel. Adequate adhesion between the hard and the soft components is important in these applications.
One of the methods used hitherto provides gaskets and damping elements separately and usually uses an additional operation for their mechanical anchoring or adhesive-bonding, thus generating additional work and sometimes considerable additional costs.
A more modern and more cost-effective method is multicomponent injection molding. In this, by way of example, a second component is injection-molded onto a previously molded first component. The adhesion achievable between the two components is of great importance for this process. Although this adhesion in multicomponent injection molding can often be further improved via introduction of undercuts within interlocking connections, good underlying adhesion via chemical affinity between the selected components is often a precondition for their use.
Examples of well-known combinations produced by multicomponent injection molding are composed of polypropylene and of polyolefin elastomers or of styrene-olefin elastomers, polybutylene terephthalate with polyester elastomers or styrene-olefin elastomers. Polyamides, too, adhere to very many soft components.
There are also known moldings composed of polyacetal onto which functional elements have been directly molded, these having been produced with use of non-crosslinked rubbers (DE-C 44 39 766). However, the bond strength of such composite bodies is not yet satisfactory.
A further publication relates to composite bodies of exactly this type which are composed inter alia of a polyacetal, of a rubber copolymer, of a reinforcing filler, of a crosslinking agent, and, if appropriate, of further conventional additives (DE-A 196 41 904). Here, the rubber component, which has been prepared in the absence of any crosslinking agent, is bonded at from 130 to 170° C., after addition of a crosslinking agent, via injection molding to a previously prepared polyacetal molding, and then the polyacetal-rubber composite body is formed in a further step at from 140 to 180° C. via vulcanization of the rubber copolymer. No particularly good adhesion of the polymer components is achieved until the rubber content has been vulcanized. However, this additional step is considered to be disadvantageous because of the elevated vulcanization temperatures and times.
DE-C 19845235 discloses composite bodies composed of polyacetal and of styrene-olefin elastomers, these having been modified via addition of non-olefinic thermoplastic material. However, disadvantages are their relatively high compression set values in various ranges of application temperature and their unsatisfactory chemicals resistance with respect to aromatic and aliphatic hydrocarbons, fats, and oils. There is a need, however, for hard/soft components which can be produced via multi-component injection-molding technology and which can be used in the engine compartments of automobiles, and which provide not only relatively low compression set values in various ranges of application temperature but also relatively high heat resistance (relatively low creep) in conjunction with improved chemicals resistance.
Another possibility in the production of composite bodies is provided by use of adherent intermediate sublayers. By way of example, EP 0 921 153 A1 discloses that polar and non-polar polymers can be mixed via use of specific block polymers as compatibilizers composed of a functionalized polymer and of a polyamide. The resultant polymer mixtures can be used as adhesive intermediate sublayer for production of composite bodies composed of a polar and non-polar thermoplastic polymer. According to EP 0 837 097 A1, production of composite bodies composed of a polar and non-polar thermoplastic polymer is also successful with the aid of block copolymers containing a chemically modified polyolefin and a thermoplastic polyurethane, a copolyester, or a polyamide. However, it would be desirable to omit use of complicated block copolymers in production of composite bodies composed of polyacetals and of thermoplastic elastomers.
It is therefore an object of the present invention to provide a composite body composed of polyacetal and of thermoplastic elastomers without the disadvantages and restrictions mentioned.
Experience hitherto in the search for novel hard/soft combinations has shown that direct combination of polyacetal and TPV is not possible (Advanced Elastomer Systems, Rev. 06/2001 p. 1; Santoprene® thermoplastic rubber 8211-55B100 TPV), since the crosslinking agents conventionally used for the TPV, e.g. phenolic resins or peroxides, lead to degradation of the polyacetal.